Stabilization of the spine is often required following trauma, tumor, or degenerative pathologies. Although each region of the spine presents unique clinical challenges, posterior fixation of the cervical spine is particularly troublesome. The anatomy of the cervical spine makes it a technically challenging area to instrument. Specifically, several vital neural and vascular structures including the vertebral arteries, nerve roots, and spinal cord must be avoided during surgery.
Current methods of posterior cervical stabilization include the use of metallic wire or cable and plate/screw systems. Both wire and plating systems suffer from limitations. For example, in certain clinical applications such as occipital-cervical and cervical-thoracic pathologies, the fixed location of the plate screw holes makes alignment of the screws difficult and may comprise the achieved stabilization.
In order to alleviate the problems associated with fixed screw hole location, devices allowing variability in screw placement are available. For example, U.S. Pat. No. 5,735,852 discloses a clamp that can be placed anywhere along the length of a rod. One problem with this and similar designs is that the clamp has a screw hole surface oriented at a fixed angle with respect to the rod. As a result, these systems often do not provide sufficient angulation of the screw to accommodate complex degenerative pathologies.
Variable angle screw systems are also currently available. U.S. Pat. No. 5,549,608 discloses a polyaxial locking screw and coupling element device for use with a rod fixation apparatus. Because the rod is located on top of the screw after the device is assembled, the device disclosed in the '608 patent has a high profile and does not allow independent screw and rod fixation. Without independent screw and rod fixation, some adjustability is lost along with the ability to compress or distract along the rod. Furthermore, should one of the components loosen, the entire construct also loosens. The osteosynthetic fixation device disclosed in U.S. Pat. No. 5,501,684 requires that the rod be threaded through a channel rather than simply slipping the rod through an opening on the side or top of the device. It should also be noted that the devices of the '684 and '608 patents were designed for use in the lumbar and sacral regions of the spine.
U.S. Pat. No. 5,643,259 discloses spine fixation instrumentation for the cervical spine. Because the instrumentation disclosed in the '259 patent relies on a sleeve mechanism to secure the rod and the screw to the connector, it is difficult to align both sleeves with the connector. Furthermore, when several connectors are used, alignment is even more difficult and unintended distraction between levels may occur when joining the rod sleeves to the connectors. Due to the limited space between adjacent vertebrae in the cervical region of the spine, there may not be sufficient room between connectors to allow placement of the sleeves. In addition, optimal orientation of the '259 patent system is limited by the inability to rotate the screw about an axis parallel to the rod, i.e. the system only provides for two degrees of freedom, possibly resulting in the need to bend the rod in order for it to fit within the connector. Since the system of the '259 patent depends on the ability to slide the sleeve mechanism along the rod when securing the rod to the connector, the fact that the rod may have to be bent to account for the missing third degree of freedom may prohibit the proper functioning of this sleeve mechanism, and the rod may not lock securely. Finally, the assembly and disassembly processes are tedious and require complicated instruments.
As the discussion above illustrates, there is a need for an improved fixation apparatus for stabilizing the cervical spine.